GB2039763A - Aeration device for fibrous suspension - Google Patents

Abstract

An aeration device for a flotation container (1) - e.g. for removing printing ink from waste paper suspension - has a centrifugal impeller (2) into the centre of which are fed the suspension and air. The impeller (1) is surrounded by stationary guide vane means (9) which form, in radial relationship to the impeller, an inner ring of guide vanes (10) inclined, in the direction of rotation of the impeller, away from the radial toward the tangential direction, and an outer ring of baffle vanes (11) inclined relative to the guide vanes (10) in a direction opposite to the rotation direction. A radially outer ring of baffle rods (12) may also be provided. <IMAGE>

Description

SPECIFICATION Inlet/aeration device for fibrous suspension The invention relates to an inlet and aeration device for introducing a fibrous suspension into a flotation container and for mixing the suspension with air, and is particularly, though not exclusively, concerned with such devices for removing printing ink from a waste paper slurry. These devices generally comprise an impeller which is provided with radial outlet openings and a central inlet for the suspension, and also a guide vane device which surrounds the impeller.

The removal of printing ink from a paper fibre slurry is generally carried out by mixing suspension with fine air bubbles on the surface of which the loosened particles of ink and dirt settle.

These air bubbles rise upwards together with the ink particles which adhere to them and can be skimmed off the surface of the slurry in the form of foam. The deposition effect is strengthened by the addition of flotation chemicals, such as e.g.

sodium soap which, in association with the water hardness is converted into calcium soap. As the air bubbles are to be as finely distributed as possible in the suspension, it has already been proposed in German Auslegeschrift 1 1 74 465 to arrange a screen basket around the impeller. The screen basket acted as a baffle plate enabling the air bubbles to be finely distributed. In addition an intimate mixing of the suspension with the air can thereby be achieved.

However, in the case of very large flotation containers, this device presents problems. On the one hand the dispersion action of perforated plates is not sufficient and on the other hand adequate conveyance of the suspension into the container with a simultaneous transport of air is not ensured. It is known that the conveying action of an impeller can be increased by incorporating guide plates if the guide plates are inclined towards the radial direction and run approximately in the direction of the exit angle of the suspension.

This produces a laminar flow with a correspondingly higher pumping efficiency.

However a disadvantage consists in that no dispersion effect or mixing of the air with the suspension occurs. In large containers adequate dispersion and mixing of the suspension with air has been achieved by incorporating improved baffle surfaces. However a disadvantage of this consists in that the pumping efficiency of the impeller would thereby suffer.

According to the present invention there is provided an inlet and aeration device for a flatation container for fibrous suspension, comprising a centrifugal impeller having in its central region an inlet for the suspension, an inner ring of guide vanes, each guide vane being inclined, in the direction of rotation, away from the radial direction toward the tangential direction, and an outer ring of baffle vanes inclined, relative to the guide vanes, in a direction opposite to the direction of rotation.

Preferably the angle of inclination of the guide vanes corresponds approximately to the exit angle of the suspension from the impeller. This measure produces a very good pumping efficiency.

In a preferred arrangement, each guide vane and the associated baffle vane are formed as a single, angled off plate. Guide arm and baffle arm thus form one unit.

For further dispersion and mixing, if desired, baffle rods with their axes substantially parallel to the impeller axis may be arranged in the outlet region of the baffle vanes.

An exemplary embodiment of the invention will now be described with reference to the accompanying drawings, in which Figure lisa longitudinal section through an impeller and a guide vane device according to the invention; and Figure 2 is a section on line Il-Il of figure 1.

On the base of a container 1 is arranged an impeller 2 which is driven via a shaft 3. The shaft 3 is surrounded by a protective tube 4 and the impeller is covered by a protective plate 5.

The fibrous suspension is supplied via a supply duct 6 leading upwards into the container 1. Air is introduced into the central region of the impeller 2 via a pipe 7. Naturally it would also be possible to effect a coarse mixing of the necessary air and the suspension in the supply pipe 6, before entry into the container 1.

The impeller 2 is provided with radially extending impeller blades 8. A ring of angled off guide plates 9 is arranged around the impeller 2. The inner portion of each guide plate 9 forms a guide vane 10, which extends parallel to the direction of the suspension from the impeller (indicated by the angle ,B). The outer part of each guide plate 9 forms a baffle vane 11. Each baffle vane 11 is angled relative to the associated guide vane 1 O, in a direction opposite to the direction of rotation. As a result the front face of each baffle vane forms a baffle surface for the emerging suspension. An underpressure zone is formed on the rear face which likewise results in a dispersion effect.

The described device provides a good pumping efficiency and a good dispersion and mixing of the suspension and air.

On the one hand the guide vanes increase the conveying action of the impeller and at the same time up to 15% by volume of air can be absorbed if necessary. The following baffle vanes ensure dispersion and uniform mixing. This is believed to take place for two reasons. Due to the described change of angle on the one hand there is formed a baffle surface for the suspension on the front sides of the baffle vanes, and on the other hand an underpressure occurs on the rear side of the baffle vanes which likewise produces a dispersion effect.

It also appears that the device aliows very large quantities to be mixed with a high air content with only a small power consumption. This improves the flotation effect whereby a paper of better quality can be produced. In order to further improve the dispersion effect baffle rods 1 2 can be arranged at regular or irreguiar intervals in the region in which the suspension emerges from the guide plates 9.

Claims (5)

1. An inlet and aeration device for a flotation container for fibrous suspension, comprising a centrifugal impeller having in its central region an inlet for the suspension, an inner ring of guide vanes, each guide vane being inclined, in the direction of rotation, away from the radial direction toward the tangential direction, and an outer ring of baffle vanes inclined, relative to the guide vanes, in a direction opposite to the direction of rotation.

2. A device according to claim 1, in which the guide vanes extend generally parallel to the exit direction of the suspension from the impeller.

3. A device according to claim 1 or 2, in which each guide vane is formed integraily with a respective baffle vane as a single, angled, plate.

4. A device according to claim 1, 2 or 3, further including baffle rods arranged radially outwardly of the baffle vanes with their axes substantialiy parallel to the impeller axis.

5. An inlet and aeration device substantially as herein described with reference to the accompanying drawings.